[Rails3]Not able to call a custom mmethod of post controller outside the post controller

I defined one of my custom method in PostsController as follows:-

class PostsController < ApplicationController
 ..开发者_StackOverflow.<<other methods truncated from display >>
  public
    def update_bid_winner (winner_id)
    @post.bid_winner_id = winner_id
    @post.save
  end  
end  

But when I try to call it from some other controller (BidsController in my case). Where Bid is a nesteded resource of post:-

resources :posts do
  resources :bids do
    member do
      get 'offer_bid'
    end
  end
 end  

I tried to call my custom method as follows from the bids controller :-

 def offer_bid
   @post = Post.find(params[:post_id])
   @bid = Bid.find(params[:id])
   @post.update_bid_winner(@bid.user_id)  <<<<<<<<<< Here goes the call
   @post.save
    redirect_to post_path(@post)
end

But I get an error saying that undefined method update_bid_winner :-

undefined method `update_bid_winner' for #<Post:0xb68114f4>  

Help me out. am I doing anything wrong here? If so , please suggest ways to achieve the same !!

Thanks in Advance.

This is not working because you are calling the method on a Post object but you have defined the method in the PostsController class. This method must be defined in the Post model file (app/models/post.rb) for it to work as you want.

Generally, methods that update an object should go in that object's respective class.

PostsController and Post are two different classes. Notice how @post is a Post object: @post = Post.find(params[:post_id])

Define the method in app/models/post.rb instead of app/controllers/posts_controller.rb.

Actually the best way to achieve my task is use the following line in the controller itself :-

@post.update_attribute(:bid_winner_id,@bid.user_id)  

No need for any new methods in model to update the attribute.
But the inputs provided as other answers really was helpful to enlighten me :). Thanks.